If HTML5 is so great, why can't a I play a single game with decent performance on the Wii U via the web browser?

It's not optimized for the Wii U.

I already told this person this, in a thread they made specifically to ask this question. Even though Wii U is HTML5 compatible, those HTML5 programs are still only designated for PC,

Based off my short use of the Xbox One internet browser, Wii U's is still better. It takes up the full screen, has the touch controls, and has a second screen. It may not seem like those features would make the intenet browser better, but they really do, IMO.